Is there a simple way to stop the mudguards on my bike rattling? They're securely connected, but when I go over bumps in the road they tend to wobble about and rattle against my front wheel.
I think it's a problem that must be solved on a one-off basis, since there so many different fender/bike combinations.
First make sure the connections are really tight. Often they work loose over time. Then try to observe what's rattling. Some heavy tape applied to points that tend to knock against the bike frame may help. Different fenders that are stiffer and have more stays is another possibility. In some cases you may simply want to cut off, eg, part of the front fender extending forward from the fork, as that area tends to be less well secured and vibrates quite a bit. (Mine cracked at the fork and the front part fell off some years back, and I haven't missed it.)
I use rubber patches from used tubes. It not always look nice, but besides dampening the rattling itself, they also dampen the sound propagation along the fender (depending of course on the amount of rubber). EVA could also be a good alternative.
Other think I sometimes have to make is to preload the metal plates which run from side to side along the fixtures (I have SKS chromoplastic fenders). If you bend them against the surface they rattle, than the "spring action" will keep the surfaces pressing against each other.
As a matter of fact, my front wheel is very silent EXCEPT when I take one hand off the handlebar (for example, to use the cycling computer or take the water bottle). Riding one-hand is even noisier than riding no-hands, because the front wheel oscilates laterally.
I think that's it, hope it helps!
I've had success with leather washers. They go between the mudguard and the screw-hole. The idea is to keep the mudguard from being able to pivot (and bang into stuff and make noise). FWIW, I run 35mm honjo fenders on 25mm conti tire.
"They're securely connected, but when I go over bumps in the road they tend to wobble about and rattle against my front wheel."
My bike developed the same problem after I put fatter tyres on the bike. The back mudguard is not noisy but the front one was. After a certain amount of 'rattling' the mudguard broke off, so let that be a cautionary tale for you! A close-fitting front mudguard can only take so much before it breaks, particularly if SKS chromo-plastic (rather than vintage steel).
Doing up the bolts and dampening them is not going to solve your problem. Only a narrower front tyre or a fatter mudguard will.
For attaching the mudguards to the fork crown and top of the seat stay, use zip ties (cable ties). ie drill or melt four holes in the mudguard, then put the zip ties through these and around the fork/stays.
In my experience, this makes a big difference to stopping the mudguards rattling. The riveted brackets on SKS mudguards soon become loose and start wobbling, zip ties are much more solid.
Plus it can give a bit more tyre clearance, which is helpful if you want to use bigger tyres on a frame without much space.
Hardware store, Plumbing section, very strong rubber washers. Might even be stronger than leather and very available. My rear fender has a clamp in the middle, that is where the old inner tube comes in. I have just found out I can use high density foam wether strip for the center fender bracket and it works great. 1/8 inch thick by 1/2 inch wide. It even has adhesive on one side to help it stay in place while installing. Just finished doing this to a Norco city glide and I can't make the fenders rattle even by hitting them.
The problem with plastic front fenders is they have little torsional strength, and most are mounted at just three points. On rough road, or when you take one hand off the handlebars, the fender will oscillate at its natural frequency, twisting about the center point mount at the fork, exacerbated if you have a substantial length of fender forward of the fork. In the worst cases the forward segment will vibrate enough to contact the tire, while the bottom edge will vibrate 180 degrees out of phase, also banging against the tire. Besides being noisy, it is dangerous. For now, I've removed my front plastic fender from my new Priority 600, swanky as it is. Think Tacoma Narrows Bridge.
This is why the zip-tie idea above should work: it changes the mount to four-points, and thus greatly reduces the torsion effect. This is also why my plastic rear fenders don't vibrate: there are two pairs of struts, and thus four points. And course there are two more center points.
My old-fashioned steel (!) front fenders on my Raleigh Sprite 3-speed never had this problem in the least, despite being 3-point mounts. Not only is the bending modulus of steel huge, but they were formed with a fancy shape and folded edges that increases the stiffness, just like corrugated panels.